A8hl)iiriier.| Oo± [Feb. 16, 



measurement of single outcrops, or scries of outcrops, — partly by g(;onictrlcal 

 construct ion of a vertical section along one line, and by means of occasional, 

 but neighboring outcrops, —and parily by construction on one or more 

 lines of observation. 



Tiie planes or horizons between the groups and formations of the Section 

 are in some cases arbitrarily assumed, being based exclusively neftlier upon 

 lilliological nor pal;eontological grounds, as the descriptive text of the 

 secliou shows. In a number of instances the division or subdivision line 

 has been necessarily drawn at the end of an exposure, or at a sudden break 

 in, or change of, the topography. 



The section must therefore be taken only for what it is worth, but by no 

 means as a complete, aulhorative or final statement of tlie constitution of 

 the Palicozoic system in Middle Pennsylvania. Many of its lueiinje will 

 no doubt be filled up by future explorers, and some of its zero points shifted 

 the better to agree with the true succession of deposits. 



iV(9. XIII. Allerjhant) Ricei- (Lower Productke) Coal Measurex. 



The Mdhoniny Sandstone at the top of the Section has commonly been 

 considered the base rock of the Barren I^Ieasures, or Lower Barren 

 Measures, of the great Bituminous Coal Field of Western Pennsylvania. 

 But as it may with equal propriety be accounted the top rock of the Lower 

 Productive Coal System (being in fact known as "The Top Rock " on 

 Broad Top), and as the Alleghany Hivcr Coal Series must be taken (as 

 Mr. Piatt's report on Somerset Co. will show) to include tlie entire 

 830 feet of Coal Measures from the Pittsburgh Coal Bed down to the 

 Potsville Conglomerate (or even lower still), no mention of Barren 

 Measures has been made in this Section. 



Tiie Mahoning Sandstone, No. 267 of the section, caps the hill to the 

 north-west of Robertsdale in the East Broad Top or Trough Cirek Coal 

 Basin, where the top of the section ends. It is given a thickness of 90 

 feet ; a portion of it may have been eroded from the surface of the hill. 



Tlie coal measures beneath it (Nos. 2GG to 251 inclusive,^ arc 174 feet 

 thick and consist of shalca, slates and sandstones, containing A work- 

 able seams of coal (2 benches each)- On account of a number of rolls 

 in the strata and the varying dip of the rocks at Robertsdale, where the 

 coals have been most extensively developed, and for the want of a sys- 

 tem of levels throughout the workings on the coal seams, it was 

 impossible, with any degree of certainty, to determine the precise inter- 

 vals between the several coal beds. The given thicknesses between 

 the coals arc only approximate, and may be found to vary as much 

 as 10 feet, although the total thickness of the series is probably as fair an 

 estimate as can be made. If the classification which is proposed for the 

 coal seams shall be verified by actual development, it gives us a means of 

 comparison witli the Alleghany series in Western Pennsylvania. 



